1. Field of the Invention
The present invention relates to an obstacle detection apparatus for a vehicle. The present invention also relates to a method of controlling an obstacle detection apparatus for a vehicle.
2. Description of the Related Art
An obstacle detection apparatus for a vehicle detects an obstacle around the vehicle using an ultrasonic sensor. The ultrasonic sensor transmits an ultrasonic wave and receives a reflected wave that is the ultrasonic wave reflected by an obstacle. Then, the ultrasonic sensor detects a presence of an obstacle and a distance to the obstacle. When the ultrasonic sensor detects an obstacle, the obstacle detection apparatus generates a warning sound.
US 2009/0009306 A1 (corresponding to JP-A-2009-14560) discloses an obstacle detection apparatus including an ultrasonic sensor and an electronic control unit (ECU). The ultrasonic sensor is disposed at a bumper, for example. The ultrasonic sensor includes a microphone having an oscillation surface. The ultrasonic sensor is configured so that a directivity and a detection distance are variable. The ECU receives signals from a speed sensor, a transmission device, and a clearance sonar activation switch. Based on the received signals, the ECU outputs a command signal to the ultrasonic sensor to change the directivity and the detection distance.
For example, when the ECU receives a signal indicating a vehicle speed from the speed sensor, the ECU compares the vehicle speed with a threshold value. When the ECU determines that the vehicle speed is greater than the threshold value, the ECU outputs a long-distance mode setting command to the ultrasonic sensor. Then, the ultrasonic sensor is set to a long distance mode and detects an obstacle. In the long distance mode, the directivity is low and the detection distance is long. In contrast, when the ECU determines that the vehicle speed is less than or equal to the threshold value, the ECU outputs a short-distance mode setting command to the ultrasonic sensor. Then, the ultrasonic sensor is set to a short distance mode and detects an obstacle. In the short distance mode, the directivity is high and the detection distance is short.
The above-described obstacle detection apparatus changes the directivity and the detection distance based on the vehicle speed. When the vehicle speed is greater than the threshold value, the obstacle detection apparatus can detect an obstacle away from the vehicle. Thus, the obstacle detection apparatus can warn early when an obstacle comes close to the vehicle. When the vehicle speed is less than or equal to the threshold value, the obstacle detection apparatus detects only an obstacle in the vicinity of the vehicle. Thus, the obstacle detection apparatus is restricted from warning that an obstacle comes close to the vehicle too early.
When the ultrasonic sensor is set to the short distance mode having the high directivity, the ultrasonic sensor is liable to detect noise compared with a case where the ultrasonic sensor is set to the long distance mode having the low directivity.
In FIG. 12A, a detection area in the short distance mode is shown by area XIla, and a detection area in the long distance mode is shown by area XIIb. When the obstacle detection apparatus performs an obstacle detection in the short distance mode having the high directivity, the obstacle detection apparatus is liable to detect noise. Therefore, the obstacle detection apparatus is liable to make a false detection in which noise is detected as a reflected wave reflected by an obstacle by error compared with a case where the obstacle detection apparatus performs the obstacle detection in the long distance mode.
In the above-described obstacle detection apparatus, the ultrasonic sensor is attached, for example, to the bumper. Thus, the oscillation surface is exposed to an outside of the vehicle. When it rains, a raindrop may adhere to the oscillation surface. If the ultrasonic sensor executes the obstacle detection in the short distance mode having the high directivity when it rains, an oscillation motion of the oscillation surface may be reduced due to a raindrop, and the directivity is further increased. A directivity of an ultrasonic sensor depends on a diameter of an oscillation surface and a frequency (wavelength) of a transmission wave, and the directivity can be decreased by increasing the frequency or decreasing the wavelength as described, for example, in DENKI ONKYO SHINDOUGAKU, Corona, 1960, pages 60-62. When the oscillation motion of the oscillation surface is reduced due to a raindrop, the ultrasonic wave is transmitted at a frequency lower than a predetermined frequency. Thus, the directivity becomes higher than a predetermined directivity.
In FIG. 12B, a predetermined detection area in the short distance mode is shown by area XIIc, and a detection area in a case where a raindrop adheres to the oscillation surface is shown by area XIId. If a raindrop adheres to the oscillation surface while the ultrasonic sensor is set to the short distance mode having a high directivity, the directivity may increase from the high directivity and the detection area may include a road surface. If the detection area includes a road surface, the ultrasonic sensor may make a false detection in which a reflected wave from the road surface is detected as a reflected wave from an obstacle by error.